Our invention relates to a method of making an incandescent lamp, particularly a baseless incandescent lamp having, instead, a hermetic pinch or press seal of fused vitreous material at one end. Typically, the baseless incandescent lamp to be fabricated by the method of our invention is of the known halogen cycle type, with a vitreous tube extending through the pinch seal for evacuating the interior of the lamp envelope and for introducing a halogen gas or the like into the evacuated envelope.
The baseless halogen-cycle incandescent lamp has been known and used extensively as, for example, a light source of vehicular headlamps. By the term "baseless lamp" we mean a lamp which includes a vitreous envelope formed by press- or pinch-scaling an open end of an envelope blank after inserting a mount assembly and a gas exhaust/supply tube in the open end of the envelope blank. The mount assembly includes a filament supported by a pair of lead wires. The gas exhaust/supply tube is used for subsequently evacuating the interior of the pinch sealed envelope and for introducing a halogen gas together with an inert fill gas into the evacuated envelope. Parts of the lead wires of the mount assembly, and part of the exhaust/supply tube, are both embedded in the pinch seal.
We will now explain the standard practice of the industry for the fabrication of such baseless lamps in order to make clear the problems to be solved by our invention.
The mount assembly and the gas exhaust/supply tube are both inserted in the open end of the envelope blank before the latter is heated. Then the open end portion of the envelope blank is heated to a moldable state, as by burners, while the envelope blank is being revolved about its own axis. As the envelope blank is thus heated, so are, of course, the amount assembly and the exhaust/supply tube inserted therein. Then the viscid end portion of the envelope blank is pinched to form the pinch seal fused to the exhaust/supply tube and also having the lead wires of the mount assembly embedded therein.
We object, in this conventional practice, to the simultaneous heating of the envelope blank and the exhaust/supply tube inserted therein. Heated to a viscid state with the envelope blank, the exhaust/supply tube has been susceptible to deformation upon pinch-sealing of the envelope blank. The tube has been easy to be clogged up or reduced in inside diameter to such an extent that a desired degree of vacuum is not created within the envelope when air is subsequently drawn out therefrom for a designated length of time.
We are aware of some suggestions heretofore made to overcome this problem. One is to insert a coil of heat-resistant wire in the exhaust/supply tube as a mandrel to prevent its deformation during pinch-sealing. Our objection to this suggestion is because of the added cost of the coil mandrel and the added task of inserting it into the tube and, after the pinch-sealing of the envelope blank, of withdrawing it for the evacuation of the completed lamp envelope.
Another known solution is to fabricate the lamp envelope and the tube from vitreous materials of improved compositions. Such improved vitreous materials are usually more expensive than ordinary vitreous materials and, moreover, impose limitations on the latitude of lamp design.
It is also known to blow a gas through the exhaust tube during the heating of the envelope blank to maintain its shape. Examples of such an approach are described in U.S. Pat. Nos. 3,932,164, 4,469,9893 and 4,749,901. This approach, however, has a drawback in that additional steps in the manufacturing process are required to connect and disconnect the tube to and from a gas supply. The efficiency of the process is thereby hindered and the costs increased.